1. Technical Field
The present invention relates to a passive equalizer such as a waveform equalization circuit, and particularly relates to a passive equalizer capable of suppressing an influence of a transmission loss and a multiple reflection in a serial transmission of an ultrafast signal exceeding 10 Gbit/second.
2. Description of Related Art
In the serial transmission of the ultrafast signal exceeding 10 Gbit/second, an influence of an output capacitance of a signal source IC and an input capacitance of a receiver IC is easily received, due to a high frequency of the signal, thereby easily generating a transmission loss and a multiple reflection which are caused by an impedance mismatch.
In addition, a pulse width of a transmission signal is dynamically changed by data content. Therefore particularly 1 unit interval (UI) signal which is minimum pulse width data generating “0” or “1” once, has a highest frequency, and is most strongly influenced by the transmission loss and the multiple reflection.
Meanwhile, signals such as 2 UI signal in which “0” and “1” are continuous twice, and 3 UI signal in which and “1” are continuous three times, are less influenced by the transmission loss and the multiple reflection, because the frequency of the signal becomes lower as the unit interval becomes larger.
Thus, in the transmission signal, decrease in amplitude which is caused by the transmission loss, and a fluctuation of a rise/fall edge which is caused by the multiple reflection, are generated in different amounts for every unite interval, thus forming a state in which an eye pattern is closed and such a state is a factor of causing a bit error.
Note that the eye pattern is the pattern obtained by repeatedly plotting a random data signal waveform in the same plot area for every 2 UI, so as to be superposed and displayed, wherein a widely opened state of an area surrounded by the rise/fall edge, namely an open state of the eye pattern is preferable.
As a means for opening the eye pattern closed by such a transmission loss, a passive equalizer as shown in non-patent document 1, namely a continuous time linear equalizer (CTLE: Continuous Time Linear Equalizer) are frequently used.
Further, as a method for recovering a deterioration of the eye pattern due to a simultaneous generation of the transmission loss and the multiple reflection, the receiver equalizer as shown in patent document 1 is proposed.
Patent Document 1: Japanese Patent Laid Open Publication No. 2010-268154
Non-Patent Document 1: “Construction of an integrated circuit system utilizing a communication/signal processing technique for Multiple-Valued Code-Division Multiple Access Techniques for Intra-Chip Communication” by Telecommunications Advancement Foundation (Research investigation report No. 23 2008, p603)
As shown in FIG. 10, non-patent document 1 teaches a passive equalizer in which a high pass filter including an inductor L21 and a capacitor C23 configured to allow only a desired high frequency component to bypass an attenuator, is respectively parallel-connected to an attenuator composed of resistances R11, R13, and R15 which are connected between an input terminal 1 and an output terminal 3. Such a passive equalizer has a relatively simple structure, and therefore can be manufactured at a low cost, and appears on the market widely.
Although the inductor L21 in FIG. 10 is omitted in the non-patent document 1, the inductor L21 actually exists because there is a residual inductance component, etc., on a connection line to the capacitor C23.
Then, a series resonant circuit is formed by the inductor L21 and the capacitor C23, and a transmission characteristic of a minimum loss at a resonant frequency of the series resonant circuit is realized.
However, even in a case of the same resonant frequency, as shown in FIG. 11, difference is generated in the transmission characteristic S21 depending on a combination of constants of the inductor L21 and the capacitor C23. When a residual component level of the inductor L21 is an extremely small value such as 0.5 nH, the characteristic is shown by curve A with a broad peak, and when the inductance value of the inductor L21 is intentionally set to be large like 5 nH, the characteristic is shown by curve B with a steep peak. FIG. 11 shows a case that the constant is set for a transmission signal of 10 Gbit/second.
A conventional passive equalizer shown in FIG. 10 has a transmission characteristic contrary to the transmission loss, and an object of such a passive equalizer is to realize a flat transmission characteristic by canceling the characteristic curve with the characteristic curve of the transmission loss, wherein the transmission loss shows a monotonously decreasing curve, and the characteristic of the curve A with broad peak is obtained as the transmission characteristic requested for the conventional passive equalizer.
However, if the passive equalizer of FIG. 10 is inserted into the transmission line in which multiple reflection occurs, it is difficult to consider that the passive equalizer of curve A is more suitable.
FIG. 12 shows an eye pattern at a receiving terminal when the passive equalizer having the characteristic of FIG. 11 is inserted into the transmission line in which the multiple reflection and the transmission loss occur simultaneously. According to FIG. 12, the eye pattern with small jitter at the rise/fall edge can be obtained in the passive equalizer of curve B.
However, the passive equalizer of curve B has unsuitable transmission characteristic for compensating the transmission loss, thereby making an opening of the eye pattern incomplete. Namely, the passive equalizer shown in FIG. 10 has a problem that there is no coexistence of the compensation for the transmission loss and the compensation for the multiple reflection.
In addition, due to a steep peak of the characteristic, the passive equalizer of characteristic B has a problem that when error is generated in the constants of the inductor L21 and the capacitor C23, the peak frequency is deviated, and the compensation cannot be done at a desired frequency.
For the reason described above, the conventional passive equalizer is easily influenced by the multiple reflection, but there is no choice to use the passive equalizer of characteristic A.
In addition, when such a passive equalizer is miniaturized, decrease of capacitance cannot not be prevented, due to a reduction of an electrode size of the capacitor C23, thus involving a problem that the conventional passive equalizer has a structure disadvantageous for miniaturization.
Meanwhile, the structure of the passive equalizer of patent document 1 is extremely simple, and an excellent waveform correction can be realized even for a high speed transmission signal of about 10 Gbit/second. However, there is an impedance mismatch state at the 1 UI frequency, and if the transmission signal of a faster transmission speed such as 16 Gbit/second or more for example is transmitted for generating a reflection wave from the passive equalizer itself, there is a possibility that the multiple reflection cannot be sufficiently removed.
For example, FIG. 13 shows the eye pattern in a case that the passive equalizer of patent document 1 is inserted into the transmission line of 16 Gbit/second in which the multiple reflection and the transmission loss are generated. Although minimum improvement of the eye pattern can be realized, this is the eye pattern having much room for improvement due to many jitters.
In order to solve the abovementioned problem, the present invention is provided, and an object of the present invention is to provide a passive equalizer suitable for a miniaturized chip component and capable of recovering deterioration of an eye pattern due to transmission loss and multiple reflection, in a ultrafast serial transmission, and particularly in a serial transmission of a ultrafast signal exceeding 10 Gbit/second.